The invention relates to a method for the transfer of heat from a heat generator to at least one heat consuming apparatus operated at temperatures above 100.degree. C. (212.degree. F.) using waste water as heat transfer agent.
The waste water in this case is, in particular, industrial waste water such as, for example, water from processes of the textile industry which contains dissolved or dispersed organic and/or inorganic substances. Some of these substances have a tendency to form incrustations on the walls of evaporation installations and can cause a decrease in the heat transfer capability and possible plugging of pipe and other conduits. Moreover, certain substances will cause corrosion of the evaporation installation at elevated temperatures. These difficulties become noticeable especially in the final evaporation phases where the concentration of these materials, and possibly also the temperatures and pressures, are elevated.
Corrosive waste waters which contain dissolved acids and/or hydrolyzing salts are found in many industries, including for example, the textile industry, metal-pickling shops, electroplating shops, etc. In particular, waste waters from the textile industry are among those industrial waste waters which contain incrustation-forming substances. For example, they may contain fiber degradation products, dyes, carbohydrates, fats, soaps and textile sizings. Other waste waters of the crust-forming type are encountered in food processing plants where waste waters containing protein or albumen are a particular problem.
According to prior art, the waste waters of the last named type have been purified heretofore in most cases by precipitation of the organic substances chemically, for example by means of polyvalent salts. However, with this type of purification, new foreign substances can be introduced. Moreover, the purification of these waste waters is possible by catalytic oxidation, ion exchange and reverse osmosis. Finally, such waste waters also can be purified biologically.
However, none of these methods satisfy the industrial requirements because in most cases the purified water can no longer be employed as consumable water, so that it must be dumped into the waterways and/or into the ground water, and this leads to ecological pollution. Additional problems are encountered in case of warm or hot waste waters, because they can be conducted into the sewage system only subsequent to a costly cooling.
The method with the best separation effect for foreign substances dissolved in water, namely the evaporation, has, up until now, received relatively little consideration because of the high energy requirements for such evaporation purification of the waste water, where water vapor would be produced at atmospheric pressure and at about 100.degree. C. and would be unusable in most cases. The energy costs would be far too high to encourage development of this method.
That is why we have already seen the development of the multistage vacuum vapor system which has been so designed that, at generally reduced energy level required for the evaporation process, a decrease in the thermal energy contained in the vapor of the final evaporation phase (by means of cooling water or in cooling towers) was still tolerable.
An evaporation method for the purification of highly contaminated industrial waste waters disclosed in the German Pat. No. 2,360,491 uses a fundamentally different procedure. According to that method the industrial waste waters are likewise used to transfer the heat from a heat source to at least one heat consuming apparatus operated at temperatures exceeding 100.degree. C. (212.degree. F.), wherein the waste water is evaporated under indirect heat exchange with the heat source under pressure in part, wherein the pressurized vapor or steam is brought into indirect heat exchange contact with the heat consuming devices, and wherein the non-volatile impurities contained in the waste water are separated with the non-evaporated portion of the waste water (waste water concentrate), and a condensate suitable for use as consumable water is obtained from the waste water steam.
With this procedure, relatively little energy is irreversibly dissipated. The high degree of purification which is accomplished with the evaporation of waste water is relatively economical and can increase the degree of profitability of a particular process. Furthermore, the purified waste water can be reused in the plant as usable water. Moreover, the incrustations of the evaporator walls, a problem under evaporation at high pressure, is, surprisingly, largely avoided.